Book - School of Science and Technology

Atypical construction features 351.21.0U value (W/m K)20.80.60.40.200 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Figure 2.5 U values for floorsFloor perimeter / Floor area (1/m)The boundary between the space to be heated and any unheated areas such as garages isincluded in the perimeter measurement but the unheated area itself is, of course, excluded.Where the perimeter/area ratio is less than about 0:3m 1 then insulation may not bestrictly necessary: however, the provision of a horizontal strip about 1 m wide around theexposed perimeter is good practice (Figure 2.8). The data plotted in Figure 2.5 have beenadjusted to relate to the air temperature difference (t ai t ao ) used for the remaining surfaces.Glass, glazing and windowsIt will be appreciated from Figure 2.1 that, since the thermal resistance of a sheet ofordinary glass is negligible, the U value for single glazing is calculated very simply from thetwo relevant surface resistances, i.e. 1/(R so ‡ R si ). Similarly, in the case of double andtreble glazing, the U value may be calculated from those same surface resistances plus, asappropriate, one or two air gap resistances. Thus, for a normal exposure; a 12 mm widesealed gap between panes and high emissivity surfaces, the notional values for glazing are:Single ˆ 1/(0:06 ‡ 0:12) ˆ 5:56 W/m 2 KDouble ˆ 1/(0:18 ‡ 0:18) ˆ 2:78 W/m 2 KTreble ˆ 1/(0:36 ‡ 0:18) ˆ 1:85 W/m 2 KThese values, it will be noted, are not very different from those in the first column ofTable 2.4. Both of the remaining examples in this same column relate to double glazingwhere one of the inner surfaces which faces the gap has been provided with a transparentlow emissivity coating ( ˆ 0:1). In the first case, a normal air gap is provided but, in thesecond, the cavity between the panes has been filled with the inert gas argon instead of air.The low emissivity coating and the argon fill each improve the thermal performance of theglazing considerably.